Gallium is an important semiconductor material and widely used. The price of gallium is very high in the international market and thus gallium has a bright prospect. However, the reserve of gallium is low, only approximately 0.015% in the earth's crust. Gallium almost does not form minerals, but exists with other minerals in form of isomorphism. Therefore, extraction of gallium is considerably difficult. Gallium is often found in conjunction with aluminum and zinc in minerals in nature. As such, sulfide deposits of zinc and bauxite ore serve as a primary source of the extraction of gallium. Nowadays, more than 90% of gallium in the world is extracted from the by-product of alumina industry in which bauxite is used as a main raw material. The mother liquid used for the enrichment and separation of gallium is the mother liquid obtained from carbon precipitation (or seed precipitation) during the process for producing alumina. The main component of such mother liquid obtained from carbon precipitation (or seed precipitation) is a base sodium metaaluminate solution containing gallium. Main methods for extracting gallium from such base solution include a method for removing aluminum via lime cream and carbonation, method of Carbonated lime milk two-stage decomposition method, precipitation method and resin adsorption method which develops in recent years.
The recent studies have shown that the fly ash obtained from some places contains a large amount of gallium which even overpasses the gallium level of mineral deposit. It has been verified by researches that the gallium content in the fly ash is usually 12-230 μg/g. As compared with the gallium contents of other resources, the fly ash deserves to be extracted for metal gallium as a raw material. In light of different conditions of calcinations, the fly ash is classified into pulverized coal-fired boiler fly ash and circulating fluidized-bed fly ash. The pulverized coal-fired boiler fly ash is produced when coal is burned at a very high temperature (1400-1600° C.), in which alumina is in glassy state or present as a mineral form of mullite crystals or corundum crystals of hot aluminum mineral which make such alumina very stable. While the combustion temperature of circulating fluidized-bed fly ash is much lower than that of traditional pulverized coal-fired boiler fly ash, only about 850° C. Different combustion temperatures make a substantial difference in phase composition between the pulverized coal-fired boiler fly ash and circulating fluidized-bed fly ash, that is, amorphous kaolinite enters into the main phase composition of the circulating fluidized-bed fly ash, in which silicon dioxide, alumina and ferric oxide or the like possess excellent activity.
CN 200810051209.5 discloses a method for extracting both alumina and gallium from fly ash. In the method, sodium metaaluminate solution containing gallium is obtained by acid-leaching and alkali-leaching processes, and then gallium is enriched and separated via multiple-stage carbon precipitation-sodium hydroxide dissolution process.
CN 200710065366.7 discloses a method for extracting silicon dioxide, alumina and gallium oxide from high-alumina fly ash. The method comprises steps of treating the residues produced after the extraction of silicon dioxide from fly ash to obtain sodium metaaluminate solution containing gallium, using such solution as the mother liquid to enrich gallium via multiple-stage carbon precipitation-sodium hydroxide dissolution process and resin adsorption process.
CN 200710145132.3 discloses a method for co-producing gallium and alumina. The method comprises steps of treating fly ash to obtain sodium metaaluminate solution containing gallium, enriching gallium by the Bayer dissolving system and then separating the enriched gallium by adsorption process using chelating resin.
CN 200710141488.X discloses a method for producing gallium. The intermediate product, i.e. mother liquid of carbon precipitation, obtained from the process for producing alumina from fly ash is used as a raw material and reacts with sodium bicarbonate, and then subjects to a thorough carbonation, so as to obtain a gallium concentrate.
In the above patent documents, the mother liquid of carbon precipitation (or seed precipitation) obtained from the process for producing alumina from fly ash is used as a raw material for the enrichment and separation of gallium, that is, the mother liquid used for extracting gallium is a base sodium metaaluminate solution containing gallium.
CN 200810017872.3 discloses a process for extracting gallium from fly ash and coal gangue. In the process, an adsorption method via absorbent columns is used for extracting gallium from an aluminum chloride solution containing gallium which is obtained by mixing fly ash and sodium carbonate, subjecting the mixture to calcination followed by water leaching and carbon precipitating and then reacting with hydrochloric acid. Such process, as fly ash and sodium carbonate are mixed and calcined at a very high temperature before acid leaching, is suitable for extracting gallium from pulverized coal-fired boiler fly ash which has weak activity.
Jiazhen He et al. has reported “a research on technique of recycling gallium from fly ash” (Scientific Research, 2002, No. 5, p23-26), in which the fly ash reacts directly with hydrochloric acid to yield an aluminum chloride solution containing gallium, without being calcined at a very high temperature, and then gallium is extracted by resin adsorption. The reaction temperature of the fly ash and hydrochloric acid is low (60° C.), which makes the leaching efficiency of gallium very low (35.2%). Moreover, the resin for extraction used in the method is levextrel resin (CL-TBP). The adsorption principle of such resin is similar to that of solvent extraction. Such resin is obtained by copolymerizing and curing the active group of an extraction agent with the base resin. Consequently, the adsorption efficiency of the resin is very low and the production cost is very high.